The First Great Schism marks the initial major division, as described in Great Schisms, between adherents of the Jedi Order and those who opposed its established hierarchy. Contemporaries at the time knew this conflict without a specific name. Later, historians designated it as the Great Schism. This changed when the emergence of the Second Great Schism made it necessary to distinguish the original conflict as the First.
For more than ten millennia, the Je'daii Order, a society of Force wielders, existed peacefully on the Deep Core planet of Tython. Subsequently, some members relocated to distance themselves from the growing influence of the nascent Galactic Republic, thereby establishing the Jedi Order. Their new base was established on Ossus, situated in the Outer Rim.
By this time, the Jedi Order had already become highly structured in its practices and, according to some, excessively rigid, thus hindering advancement. This viewpoint was notably held by Xendor, a Kashi Mer Jedi Knight, who faced exile from his home for embracing the darker aspects of the Force and sought to deepen his understanding of Bogan. He openly criticized what he saw as the Jedi Order's increasingly restrictive approach to Force usage. He formally requested permission from the Jedi Order's leadership to leave Ossus and create a new academy. His vision was to teach and explore alternative traditions of the Force, including Dai Bendu, the Palawa, the Way of the Dark, and the Protectorate of the Hidden, among others. Despite the denial from the Jedi Order's leaders, Xendor defied their decision and departed. He gained the support of Arden Lyn, a young Jedi who shared his perspectives and felt disillusioned with the Jedi Order's adoption of Caamasi philosophy.
Xendor and Lyn left Ossus to found a new academy located on Lettow. They were not alone in their defiance of the Jedi Order's rules; increasingly larger groups abandoned Ossus to join Xendor's nascent group and delve into Force mysteries that were forbidden under the Jedi Order. Eventually, Xendor's followers became known as the Legions of Lettow.
The new arrivals at Xendor's academy pledged their loyalty to him and to the protection of their new order. They vowed to defend their right to pursue knowledge of the Force without the oversight of the Jedi hierarchs, whom they considered unqualified to dictate which paths were correct and which were forbidden. Members of the Legion embraced the dark side of the Force, rejected the idea of order and structure and celebrated their freedom by giving in their passions and emotions and touching the power of the dark side.
Accounts differ regarding which side initiated the conflict. Jedi records and some historians claim that Xendor and his army, driven by rage, launched an attack on the Jedi Order. Conversely, others, including Arden Lyn, assert that the Jedi declared war against the Legions and mobilized an army first.
One of the initial battles took place on Ossus, the Jedi's capital world. Xendor, claiming a desire to quickly end the conflict and prevent its escalation into the Republic, led the Legions in an assault on Ossus. This attack was unsuccessful, and the war continued without abatement.
Millennia later, Lyn recounted that Xendor attempted to warn the young Republic about the threat he believed the Jedi posed, suggesting they would present themselves as protectors while secretly plotting to seize power. However, Xendor's efforts were futile, and the Legions failed to gain any support within the Republic against their Jedi adversaries.
As the conflict intensified, it spread across numerous worlds. Battles erupted on Chandrila, Brentaal, the Republic capital Coruscant, and Metellos, culminating in a significant battle on Columus. At Columus, Xendor faced the Jedi Awdrysta Pina, known as Green Blade, in single combat and was killed.
Lyn later reflected that both she and Xendor had admired Pina for his independent thinking, as he often ventured to unknown places in search of Force mysteries beyond the Jedi Order's knowledge. However, Pina ultimately believed that order and structure were the foundations of civilization, leading him to side with the Jedi against Xendor.
The Jedi forces under Pina's command possessed a unique advantage over the Legions, which even Lyn acknowledged: their dedication to order allowed them to subsume their individuality into a larger collective and fight as one—not unlike the Force ability known as battle-meld. Xendor's followers refused to embrace any such tactic, believing it insectile and an affront to individuality.
Despite the victory over Xendor, the Jedi did not declare the war over. Instead, forces under Pina's command were dispatched to Lettow with the goal of exterminating the remaining members of Xendor's faction. A fierce battle ensued, providing just enough time for the last survivors of the Legions to escape beyond the borders of Republic space.
Lyn was eventually cornered by Pina on Irkalla, after fleeing into regions of space unknown to all but Xendor. The two engaged in a duel during which she was disarmed. Unwilling to concede defeat, she channeled her strongest emotions—rage and sorrow, love for Xendor, and despair over what she believed had been unfairly taken from them—through the Kashi Mer talisman, destroying Pina's sword and mortally wounding him in the process.
In his final moments, he utilized his unique Force ability, Morichro, to defeat Lyn. This technique essentially placed her in a state of deep stasis. She remained in this state until the era of the Great Jedi Purge, over 24,000 years later, when she was revived.
Following the Great Schism, the Jedi Order retreated to Ossus. Subsequently, the Jedi faded from the minds of many in the Core Worlds. Unbeknownst to them, however, the Jedi secretly acted as the Republic's defenders in the Rim, working quietly to curb the ambitions of the Hutts towards vulnerable colonies in the Slice, and maintaining division among the militaristic states of the Tion Cluster. They did not publicly re-engage with the Republic until the Tionese War, nearly 500 years later.
In the comic book Tales of the Jedi - The Golden Age of the Sith #0, published on July 31, 1996 by Dark Horse Comics, it is mentioned that "The First Great Schism" initiated "a century of bloodshed" that resulted in the Dark Jedi Exiles founding the Sith Empire. In Tales of the Jedi – The Golden Age of the Sith #2, the Dark Lord of the Sith Marka Ragnos states that the ancestors of the Sith Lords were exiled due to "The Great Schism", implying that because his ancestors were exiled at the end of the "First Great Schism", he is unaware that there was a "Second Great Schism".
The Emperor's Pawns, a collection of biographies by Abel G. Peña published in Star Wars Gamer #5 (2001), revealed that Xendor and his lover Arden Lyn initiated the First Great Schism around 25,000 BBY. The lore surrounding the First Great Schism from Tales of the Jedi – The Golden Age of the Sith #0 and the date from The Emperor's Pawns were reiterated by the timeline at the beginning of and Naga Sadow's entry in The New Essential Guide to Characters (2002).
The New Essential Chronology (2005) revealed that the Jedi Exiles existed around 7000 BBY, and that the preceding century of conflict is the same event as the Hundred-Year Darkness (previously presented as a separate event in The New Essential Guide to Characters timeline) and followed the "Second Great Schism." The First Great Schism was retconned into a separate event. When questioned about this retcon, Dan Wallace, the author of The New Essential Chronology, explained that Lucasfilm internal dating documents indicated that the Jedi Exiles made contact with the Sith species around 7000 BBY. Leland Chee stated that this retcon aimed to "correct the the (sic) original discrepency of the timing of the Great Schism introduced by The New Essential Guide to Characters", but did not specify which source The New Essential Guide to Characters created a discrepency with.